Hydrogeologic Framework of the
Floridan Aquifer System in Florida and in Parts of Georgia, Alabama, and
South Carolina

By

James A. Miller
Professional Paper 1403-B

ABSTRACT

The Floridan aquifer
system of the Southeastern United States is comprised of a thick sequence
of carbonate rocks that are mostly of Paleocene to early Miocene age and
that are hydraulically connected in varying degrees. The aquifer system
consists of a single vertically continuous permeable unit updip and of two
major permeable zones (the Upper and Lower Floridan aquifers) separated
by one of seven middle confining units downdip. Neither the boundaries
of the aquifer system or of its component high- and low-permeability zones
necessarily conform to either formation boundaries or time-stratigraphic
breaks.

The rocks that make
up the Floridan aquifer system, its upper and lower confining units, and
a surficial aquifer have been separated into several chronostratigraphic
units. The external and internal geometry of these stratigraphic units is
presented on a series of structure contour and isopach maps and by a series
of geohydrologic cross sections and a fence diagram. Paleocene through middle
Eocene units consist of an updip clastic facies and a downdip carbonate bank
facies, that extends progressively farther north and east in progressively
younger units. Upper Eocene and Oligocene strata are predominantly carbonate
rocks throughout the study area. Miocene and younger strata are mostly clastic
rocks.

Subsurface data
show that some modifications in current stratigraphic nomenclature are necessary.
First, the middle Eocene Lake City Limestone cannot be distinguished lithologically
or faunally from the overlying middle Eocene Avon Park "Limestone."
Accordingly, it is proposed that the term Lake City be abandoned and the
term Avon Park Formation be applied to the entire middle Eocene carbonate
section of peninsular Florida and southeastern Georgia. A reference well
section in Levy County, Fla., is proposed for the expanded Avon Park Formation.
The Avon Park is called a "formation" more properly than a "limestone"
because the unit contains rock types other than limestone. Second, like
the Avon Park, the lower Eocene Oldsmar and Paleocene Cedar Keys "Limestones"
of peninsular Florida practically everywhere contain rock types other than
limestone. It is therefore proposed that these units be referred to more
accurately as Oldsmar Formation and Cedar Keys Formation.

The uppermost hydrologic
unit in the study area is a surficial aquifer that can be divided into (1)
a fluvial sand-and-gravel aquifer in southwestern Alabama and westernmost
panhandle Florida, (2) limestone and sandy limestone of the Biscayne aquifer
in southeastern peninsular Florida, and (3) a thin blanket of terrace and
fluvial sands elsewhere. The surficial aquifer is underlain by a thick sequence
of fine clastic rocks and low-permeability carbonate rocks, most of which
are part of the middle Miocene Hawthorn Formation and all of which form
the upper confining unit of the Floridan aquifer system. In places, the
upper confining unit has been removed by erosion or is breached by sinkholes.
Water in the Floridan aquifer system thus occurs under unconfined, semiconfined,
or fully confined conditions, depending upon the presence, thickness, and
integrity of the upper confining unit.

Within the Floridan
aquifer system, seven low permeability zones of subregional extent split
the aquifer system in most places into an Upper and Lower Floridan aquifer.
The Upper Floridan aquifer, which consists of all or parts of rocks of Oligocene
age, late Eocene age, and the upper half of rocks of middle Eocene age,
is highly permeable. The middle confining units that underlie the Upper
Floridan are mostly of middle Eocene age but may be as young as Oligocene
or as old as early Eocene. Where no middle confining unit exists, the entire
aquifer system is comprised of permeable rocks and for hydrologic discussions
is treated as the Upper Floridan aquifer.

The Lower Floridan
aquifer contains a cavernous high-permeability horizon in the lower part
of the early Eocene of southern Florida that is called the Boulder Zone.
A second permeable unit that is cavernous in part, herein called the Fernandina
permeable zone, occurs in the lower part of the Lower Floridan in northeastern
Florida and southeastern Georgia. Both these permeable zones are overlain
by confining units comprised of micritic limestone. The confining unit that
overlies the Boulder Zone is of subregional extent and is mapped as a separate
middle confining unit within the Lower Floridan.

Major structural
features such as the Southeast and Southwest Georgia embayments, the South
Florida basin, the Gulf Coast geosyncline, and the Peninsular arch have
had a major effect on the thickness and type of sediment deposited in the
eastern gulf coast. The effects of smaller structures are also evident.
For example, the Gilbertown-Pickens-Pollard fault system in Alabama locally
forms the updip limit of the Floridan aquifer system. The series of grabens
that comprise the Gulf Trough of central Georgia serves as a low-permeability
barrier to ground-water flow there. These Gulf Trough faults have downdropped
low-permeability rocks opposite permeable limestones to create a damming
effect that severely retards ground-water movement across the fault system.
Their effect can be seen on potentiometric surface maps of the aquifer system.
Other small-displacement faults in peninsular Florida do not appear to affect
the regional flow system because there is no apparent change in the permeability
of the rocks that have been juxtaposed by fault movement.

Variations in permeability
within the Floridan aquifer system result from a combination of original
depositional conditions, digenesis, large- and small-scale structural features,
and dissolution of carbonate rocks or evaporite deposits. Local permeability
variations are accordingly more complex than the generalized regional portrayal
presented in this report.

(The entire report
is available below)

Information about on-line reading and printing of historic documents

These
reports and documents have been scanned from the original hard-copy materials
and are made available on the internet in both HTML and PDF formats. Because
these are scanned documents, we are unable to provide fully-accessible versions
of these reports. If you cannot fully access the information in these documents,
please contact Heather S. Henkel at hhenkel@usgs.gov.

The
HTML versions of these documents have been created to provide the information
in a format that is quickly and easily readable over the Internet. Selective
pages and images can be printed from this HTML version by placing the cursor
inside the right-hand frame and selecting the print option from the browser.
The PDF version of the reports are also provided, and are the recommended
format to use for the best printer format and resolution.

Please note - some PDF files are very large (over 2 MB) and may take some
time to download, depending on your system.